The present invention relates to a piston with a metal shaft and a piston head that is attached to it with a shrink ring, that faces the combustion chamber, that can if necessary have a combustion pan, and that has a member made of partially stabilized zirconium oxide. The shaft can have a pair of gudgeon-pin bosses or other means of attaching a connecting rod.
A piston with a member made of partially stabilized zirconium oxide (PSZ) that constitutes part of the head is known from "Piston Design for High-Combustion Pressures and Reduced Heat Rejection to Coolant" by Wacker and Sander. The head described in that article is attached to the piston with a steel ring shrunk around the partially stabilized zirconium-oxide member and screwed to an aluminum piston shaft. One drawback of this type of piston is that the side of the head that faces the combustion chamber does not consist entirely of the zirconium-oxide member, whereas the ring forms a heat-sink bridge. Another and decisive disadvantage that is described, however, is that high thermally induced stresses occur as the result of the properties of the zirconium oxide and lead to destruction of that member.
"PSZ Ceramics for Adiabatic Engine Components" by Woods and Oda describes another piston with a member made of partially stabilized zirconium oxide. This member, which has a combustion pan, has a collar shrunk at the edge into an iron piston shaft. Since the shaft is attached flush to the collar, the drawback of insufficient insulation occurs here as well because the edge of the shaft constitutes a heat-sink bridge. Another disadvantage of this piston is that it can be manufactured essentially only out of iron because its thermal expansion must correspond to that of zirconium oxide. Most steels and aluminum, which is often used in such applications, are inappropriate because their thermal expansion is higher.
Although one particular development of this proposal, in which the metal piston shaft is covered by the collar of the partially stabilized zirconium-oxide member and attached to the section of the member below the collar with a shrink component, does completely insulate the shaft, the shrunk connection does very little to prevent the member from coming loose.